1. Field of the Invention
A method and an apparatus for finding the location of one or more holes in a fluid flow system such as a pipe, duct, or conduit, using one or more tracers that interact with or can be chemically or physically differentiated from the liquid or gas contents located outside the system and pulled back into the system when the system is placed under a vacuum. The invention is particularly applicable to underground pipes. This invention is related to the invention disclosed in U.S. patent application Ser. No. 10/960,653, filed Oct. 6, 2004, titled Method and Apparatus for Detecting and Locating Explosives, Biological, and Chemical Substances in Ducts and Structures Using Tracers.
2. Brief Description of the Prior Art
Locating holes in piping, especially underground or buried piping is a challenging and important problem. It has both environmental and economical implications. A method for accurately locating a leak in underground pressurized piping is of immediate benefit to the petroleum, chemical, and nuclear industries.
This specification describes a method and an apparatus for finding the location of one or more holes in a fluid flow system such as a pipe, duct, or conduit, using one or tracers that are injected into the system and that interact with or can be differentiated chemically or physically from the liquid or gas contents outside the system when pulled back into the system under a vacuum. PCUT is an acronym for Pipeline Characterization Using Tracers, and characterization includes detection, location, and quantification of a contaminant in the pipe. The PCUT technology was demonstrated in a pipe using a thin layer of diesel fuel trapped at the bottom of the low point of the pipe as the contaminant. The method of the present invention differs from the method for detecting and locating contamination in a pipe, because the contamination was already present in the pipe. In the present invention, a vacuum is generated in the pipe to pull in the released liquid (or gas) contents of the pipe that was released or leaked through a hole in the pipe wall. Once the released contents are pulled back into the pipe, the PCUT technology applies, using the released contents that were pulled back into the pipe as a marker of the location of the leak hole.
There are a number of methods used to locate a hole in a pipe. These methods have been used to find holes in pipelines containing petroleum fuel, natural gas, dangerous chemicals, and radioactive liquids. The most conventional approach is to insert a tracer into the pipe and search for it outside the pipe. If the tracer is detected, and if it is unique (i.e., not part of the background environment), the presence of the tracer outside the pipe is an indication that the pipe has a hole. The location of the tracer is an indication of the location of the hole. For example, helium is often used as a tracer.
When the pipe is buried underground, which is the most common need for the method of the present invention, a carefully designed sampling procedure is required to detect the presence of the tracer. While liquid tracers can be used, gaseous tracers or liquid tracers that volatize when released into the ground are the most often used, because gaseous tracers rise to the ground surface where their presence can be detected. Closely spaced vapor samples along the length of the pipe with a hole are required. Liquid tracers are not as common or cost effective for this application, because once the tracer is released into the soil, it is not easily sampled. The use of gaseous tracers is expensive to implement and has many technical pitfalls. It is expensive to implement, because samples must be collected at 5- to 20-ft intervals to avoid missing the presence of the tracer, and the tracer material is generally expensive. Technically, the location of the underground pipe must be well known for the method to work. If the samples are too far off the centerline of the pipe, the presence of any tracer can be missed. A larger problem is that the subsurface soil is not homogenous and the tracer will tend to follow underground pathways and rise to the surface off the centerline of the pipe. Finally, the performance of the method depends on the nature of the tracer. For best performance, the tracer must be unique and not be present in the environment. In some cases, constituents in the pipe fluid are used. These have not been very effective because previous releases and spills of the pipe contents contaminate the soil. Adding a tracer to the pipe that is unique and is not present in the pipe fluid or the surrounding soil is the most effective. However, both types of tracers have resulted in false alarms because of their presence in the surrounding soil. While there are technical pitfalls, this technique has been widely applied.
Another approach is to use passive acoustics. Two acoustic sensors are attached to the pipe in such a way that the sensors bracket the hole. The pipe is then pressured so that an acoustic signal is generated at the hole. This technique will work with both liquid- and gas-filled pipes. However, the acoustic leak signal generated in a gas-filled pipe is much weaker than the acoustic signal generated in a liquid-filled pipe. Advanced signal processing using correlation and coherence analysis algorithms are required for accurate location estimates. The distance between the sensors on the pipe, and a good estimate of the acoustic propagation velocity are required to make accurate location estimates. In general, for small holes, sensor separations must not be too much greater than 100 ft for gas-filled pipe and 500 ft for liquid-filled pipe. Also, access to the pipe is required to mount the sensors on the pipe. This can be expensive if the pipeline to be interrogated is long and the pipe has to be excavated at many locations to obtain the required spacing to implement the method. This technique is also widely used, but is best used in pipe applications where the pipe is on the order of 500 to 1,000 ft or where access to the pipe exists (e.g., at valve pits).
There are other approaches to locating a hole in a buried pipe. There are infrared sensing methods that detect the presence of the released pipe contents from a change in temperature between the contaminated and uncontaminated regions. This approach has not been very effective.
There are other approaches to locating a hole in a buried pipe. There are infrared sensing methods that detect the present of the release pipe contents from a change in temperature between the contaminated and uncontaminated regions. This approach has not been very effective.
There is a need for a method that does not require sampling at closely spaced intervals at the surface or excavation to the top of the pipe for implementation of the method. There is a need for a method with higher performance than the tracer, acoustic and infrared systems now in use. Stated differently, there is a need for a method that can be used on long sections of pipe that is both more reliable and more accurate than these three methods. Finally, there is a need for a method that is less expensive than these current methods.
The method of the present invention uses a gaseous tracer method developed by the inventors to locate a hole in a pipe. While the invention can also detect the presence of a hole, there are more reliable, accurate, less expensive methods, and commercially available methods, such as volumetric and pressure-based methods, that are used that can perform this task. These detection methods outperform and are less expensive than the tracer, acoustic, and infrared methods mention above. The method is called PCUT (Pipeline Characterization Using Tracers). A gaseous tracer that interacts with the contaminant of interest is used. Both partitioning and reactive tracers can be used for this application. The method described here is very similar to the one described in two patent applications submitted by the inventors for characterizing (detection, location, and quantification) contamination in pipes, ducts, and other fluid flow systems, and for find dangerous and hazardous substances like explosive devices in ducts, buildings and the like. However, the application objective is very different, but the technology for location, which is required for this application, is very similar.
FIG. 1 is a simplified illustration of the use of PCUT for detecting and locating the presence of a liquid contaminant, such as diesel fuel, in a pipe, using a conservative tracer and two partitioning tracers. As illustrated in FIG. 1, the method was implemented by injecting a “slug” of two tracers 120 and 130 into a pipe 10 or other fluid flow system, with different partitioning coefficients (Ki). One of the tracers is a conservative tracer, i.e., it will not dissolve, adhere, or interact with the hazardous substance of interest. The other tracers are selected so they will dissolve, adhere or interact with the hazardous substance of interest. The tracers are transported or advected from the injection point 150 (at one location in the pipe) to one or more extraction points 250 (at other locations in the pipe) by a gas flow field established in the pipe prior to the injection of the tracers 120 and 130. The gas flow field used to transport the tracers is typically nitrogen or air, because they do not generally interact with the tracers or the hazardous substances in the fluid flow system. The velocity of the advection flow field is selected so that the tracers have enough time to fully dissolve, adhere or interact with the hazardous substance before the leading edge of the tracer reaches the extraction point and is measured with a gas chomatograph (GC) or some other sensing means that can measure the magnitude of the tracers reaching the measurement point 250. At that point, no more tracer is introduced into the line. By measuring the time history of the concentration 70 of the partitioning 72, 74 and the conservative 76 tracers at the extraction point in the pipe, the presence and amount of the contaminant within the pipe or duct can be determined.
Detection and quantification can be accomplished using the difference in the mean arrival time of the partitioning and conservative tracers, or the difference in the levels of concentration between the conservative and partitioning tracers. The location of the contaminant can be determined by introducing a perturbation to the advection flow field. This can be accomplished by flushing (i.e., removing) the conservative and partitioning tracers in the line, and then measuring the mean time of arrival of the partitioning tracers that are still being eluted from the contamination in the system. Alternatively, this can be accomplished by introducing enough partitioning tracer at the beginning of the duct test to cover the entire duct, then stop the flow, and allow the tracer to interact with dangerous or hazardous substance. After a period of time, an advection flow field is established, and GC samples are collected and analyzed.
A series of over 25 laboratory experiments were conducted to demonstrate the capability of PCUT for detecting, locating, and quantifying a contaminant using one or more interactive tracers, and for some measurements, one or more conservative tracers. The location capability of the PCUT technology was demonstrated in the laboratory using the 116-ft, long-pipe illustrated in FIG. 12. The long pipeline 900 (116 ft) is comprised of 66-ft of 2-in.-diameter pipe 910 between the inlet 902 and the first 4-ft 3-in.-diameter section of PVC pipe 912 and 45-ft of 2-in. PVC pipe 914, 916, 918 between the first 3-in.-diameter PVC pipe section 912 and the outlet 904. The second 3-in.-diameter PVC pipe section, which could be used as a contamination point, was not used in the location tests presented herein.
A 3-ft by 1.625-in. rectangular tray was inserted into the 4-ft section of 3-in. diameter PVC pipe 912 whose center position was located 47.5 ft from the outlet end of the pipe 904 where the GC measurements were made. The shallow tray was used to hold 300 ml of aged diesel fuel. Two partitioning tracers (C7F14 and C8F16) were used for these tests. The advection gas was nitrogen.
FIG. 13a shows the time history of the concentration curves of the two partitioning tracers 392, 394 and the conservative tracer 390 from one of the location tests (Pipe Test #8). Superimposed on these curves is the flow rate of the advection gas 385, 387, 389. The location measurement is made after the detection measurement. The data required for detection is the same as for previous tests except only enough data needs to be collected to define the peak of the conservative tracer. This allows a comparison between the partitioning tracers and the conservative tracer for detection and also allows sufficient time for the tracers to partition into the contaminant. The next step is to rapidly flush 387 the conservative and partitioning tracers through the pipe and then to re-establish the advection flow stream 388 at a known velocity 389. As shown in FIG. 13a, the line was flushed at 350 ml/min 387, which is over 10 times the initial flow rate of the measurements 385. Once the flow rate is re-established 388 at a flow rate of approximately 20 ml.min 389, the partitioning tracers 393, 395 in the diesel fuel re-enter the flow stream and are advected to the end of the line at a known flow rate 389. The location of the contamination was then determined from the advection velocity 389 and the arrival time of the tracers. The arrival time is the time between the end of the flush 388 at the high flow rate 387 and the arrival of the partitioning tracers 393, 396 being emitted from the contaminant. FIG. 13b shows the two partitioning tracers 398, 399 arriving at about 63 hours; the advection flow field 397 was re-established at 44 hours 396.
Table 1 summarizes the location results for the tests conducted in the long pipe 900 illustrated in FIG. 12. Two methods were used to locate the contamination. Both methods used the time of arrival of the leading edge of the tracer concentration curves. (The average time of arrival can also be used.)
TABLE 1PCUT estimation of the location of the 300 ml of diesel fuelcontamination.Method 1Method 2TestLocation (ft)Error (%)Location (ft)Error (%)Pipe Test #8 51.33.3%53.24.9Pipe Test #1038.16.4%Pipe Test #1245.40.1%Pipe Test #2344.70.6%Average2.6%* The actual location of the contamination is centered 47.5 ft from the outlet end of the 116-ft pipe.
The first method, which requires a priori information about the diameter or geometry of the pipe, uses the maximum velocity of the advection fluid within the pipe and the time of arrival of the tracer(s) after flushing. The average velocity is computed by dividing the measured volumetric flow rate by the diameter of the pipe; the maximum velocity (for laminar flow in a pipe) is twice this value. The second method, which does not require a priori information about the diameter or geometry of the pipe, only the total length of the pipe, utilizes the ratio of the time of arrival of the leading edge of the first tracer pulse 392 or 394, which traveled over the full length of the pipe (i.e., 116 ft) 900 and the time of arrival of the second tracer pulse 393 or 395, which traveled only the distance from the contamination 912 to the outlet end 904 of the pipe 900. After weighting the arrival times by the mean of the measured flow rates, the distance from the outlet end 904 of the pipe to the contamination 912 can be determined. Both methods were applied to Pipe Test #8 and only the first method was applied to Pipe Tests # 10, #12, and #23.
The location test was also repeated (Pipe Test #12) using a dried glue sample of approximately 20 grams (10 ml). For this test the pipeline was flooded with tracer overnight and then flushed with 350 ml/min of the advection gas. After the flush, an advective flow stream was established and used to determine the location of the dried glue specimen. The distance from the end of the pipe to the glue sample was calculated to be 49.2 ft which is less than 10% error on the actual value of 47.5 ft.